The present invention relates to a method for drying wood plates or panels using hot plates for holding the wood plates therebetween, and an apparatus for carrying out this method. Wood plates dried by the method and apparatus according to the present invention are mainly veneers used for making plywood. However, wood plates intended herein are not limited to veneers, because any other type of wood plates that can be dried while being held between hot plates also can be dried by the method and apparatus according to the present invention.
A hot blow drying method is widely used for drying wood plates obtained by slicing or cutting lumber. Such hot blow drying method includes transferring veneers by a conveyor and drying the same with hot blow, i.e. hot blown air or other gas. This method has such a drawback that veneers dried by this method are warped significantly. Consequently, this method cannot be used for drying veneers which are susceptible to being warped. A method for drying veneers held between respective hot plates does not have such drawback. Recently, raw woods such as lauan have become difficult to obtain. As a result, small diametered lumber of low cost that is susceptible to being warped after being processed to wood plate and dried and that therefore hardly has been used in the past, now is required to be dried at low cost. Under such circumstances, a method for drying wood plates using hot plates has attracted more attention than has a method for drying wood plates with hot blow.
A method for drying wood plates by using hot plates for holding the respective wood plates therebetween has the following advantages in comparison with a method for drying wood plates with hot blow:
1. Little heat energy is necessary. PA1 2. Deformed wood can be ironed and made into a flat plate through drying, and therefore miscellaneous trees can be processed into veneers used for making plywood. PA1 3. All of a wood plate can be dried uniformly. PA1 4. Rough peeled wood surfaces can be altered. PA1 5. The area contraction ratio or amount during drying can be reduced. PA1 6. Suspension of operation due to accidental fires can be reduced. PA1 1. Hot plates arranged in multistep manner are opened, and wood plates are fed into the spaces between respective pairs of adjacent hot plates. PA1 2. Then, the hot plates are closed to press the wood plates therebetween. This condition is maintained for a predetermined time, and thereby the wood plates in the pressed condition are heat-dried by the hot plates. PA1 3. When drying of the wood plates is completed, all the hot plates are opened. Then the dried wood plates are discharged, and new undried wood plates are fed into the respective spaces between the hot plates.
This method has the above mentioned advantages since the entire surface of each wood plate is pressed and heat-dried by hot plates.
On the other hand, according to a method for drying lumber with hot blow, moisture is removed by evaporation from a lumber I as shown in FIG. 1. At this time, the percentage of moisture content of the lumber and the temperature change are shown in FIG. 2. In FIG. 2, the region A indicates lumber preheating, which means the preliminary time required for raising the temperature of the lumber to the wet-bulb temperature of the hot blow. The region B indicates constant rate drying during which all of the heat supplied by the hot blow is used for evaporating moisture of the lumber while maintaining constant the temperature of the lumber as long as moisture exists in the surface portion of the lumber. During such constant rate drying the water content of the lumber decreases in proportion to time. The region C indicates decreasing or falling rate drying during which the movement of the moisture from the inside of the lumber to the surface cannot catch up with or match the evaporation of the moisture from the surface. Thus, the surface portion of the lumber is dried and the temperature of the lumber surface is raised. The movement of the moisture is delayed and therefore the drying rate becomes low, i.e. it is falling.
According to such a hot blow drying method comprising the above mentioned steps practically employed at present in plywood factories, it is regarded that 1500 kg of steam is required for drying 1 m.sup.3 of lumber, and 2.5 kg to 3 kg of steam is required for drying 1 kg of moisture. On the other hand, according to a method for drying wood plates using hot plates, only about 800 kg of steam is required for drying 1 m.sup.3 of lumber and only less than 2 kg of steam is required for removing 1 kg of moisture. Thus, the necessary quantity of steam can be remarkably decreased. Further, according to a hot blow drying method, the necessary electric power for the drying operation is about 70 kw/m.sup.3. On the other hand, according to a method for drying wood plates using hot plates, the necessary electric power for the drying operation is as low as about 24 kw/m.sup.3.
A method for drying wood plates using hot plates having the above mentioned advantages is disclosed in Japanese Non-examined Patent Publications No. 202988 issued Sep. 7, 1987 and No. 299689 issued Dec. 26, 1987. Such methods include the following steps.
As above mentioned, according to this method, wood plates are fed into and discharged from all the spaces between the respective pairs of opened hot plates. Therefore, an apparatus for carrying out this method requires a loader which is a multistep type feeding means for feeding a plurality of wood plates into the spaces between the respective opened hot plates at the same time, and an unloader which is a multistep type discharging means for discharging the plurality 6f dried wood plates at the same time. The loader and the unloader are disposed on opposite sides of the apparatus. The loader feeds wood plates into the spaces between the opened hot plates from one side of the apparatus, and the unloader discharges the dried wood plates to the other side. As a result, the entire apparatus is fairly large-sized. Further, each of such loader and unloader is of a complicated and expensive structure. Therefore, the drying line is substantially expensive. Further, since the loader and the unloader feeds or discharges a number of wood plates at the same time, it is very difficult to transfer all of the wood plates to correct positions. If only one wood plate is not correctly transferred during a transfer operation, the entire apparatus must be stopped. Further, since a number of wood plates are fed to a number of positions, if the wood plates are jammed, substantial time is required to correct the positions thereof.
Further, neither the conventional apparatus for drying wood plates using hot plates nor the conventional hot blow drying apparatus for wood plates can dry each of a plurality of wood plates to the same percentage of moisture content. This is because all of the wood plates are dried under the same conditions. When wood plates of high moisture content and of low moisture content are dried at the same time, the moisture content of all the wood plates decreases. However, the wood plates of high moisture content will still have a relatively high moisture content when the wood plates of low moisture content are dried sufficiently to have a desired low moisture content. When dried wood plates still have an excessively high moisture content they must be dried again. However, almost all plates that are dried twice are over dried. When veneers of different moisture contents are adhered together to make a plywood, the obtained plywood tends to warp. When veneers of different moisture content are adhered together, all the veneers gradually come to have the average moisture content rate. During this stage, the veneers of different moisture content contract at different respective contraction rates, and consequently the plywood warps. If wood plates transferred to the drying apparatus are made to have the same moisture content, then the dried wood plates will have the same moisture content. However, it is impossible to ensure that all wood plates before being transferred will have the same moisture content. For example, the moisture contents of veneers used for making plywoods vary from 70% to 300%. Further, if veneers are obtained from the same wood material source, one veneer obtained from a central part of the wood material source and another veneer obtained from a circumferential part thereof will have different moisture contents.
Furthermore, the conventional apparatus for drying wood plates using hot plates and the conventional hot blow drying apparatus cannot dry wood plates of different thicknesses at the same time. Since they are dried under the same conditions, the thin wood plates are over dried and the thick wood plates are dried insufficiently. Consequently, it is necessary to change the drying conditions according to the thickness of the wood plate to be dried. However, in order to change drying conditions according to the thickness of the wood plate to be dried, much time and labor are required. In such case, wood plates cannot be successively dried, which causes lowering of operation efficiency. Therefore, when wood plates of various thicknesses are dried, the operation efficiency of the apparatus as a whole is lowered remarkably.
The present invention has been developed in order to eliminate these drawbacks of the conventional method and apparatus.
An important object of the present invention is to provide a method for drying wood plates using hot plates, which method can efficiently dry a number of wood plates at low cost.
Another important object of the present Invention is to provide a compact and inexpensive apparatus for drying wood plates using hot plates, which apparatus can dry wood plates of different moisture contents, thicknesses and the like, so as to have the same moisture content when dried.